Electromagnetic actuating means for a shutter mechanism in an electron microscope



Dec. 19, 1967v G. F. BAHR ET AL 3,359,418

ELECTROMAGNETIC ACTUATING MEANS FOR A SHUTTER MECHANISM IN AN ELECTRONMICROSCOPE Filed Dec. 11, 1964 -Flc;.5

EE F. gm n OTMAR h. SA 4ERLaTZKy EL/WAR 25/7152 ATTORNEY United StatesPatent ELECTROMAGNETIC ACTUATING MEANS FOR A SHUTTER MECHANISM IN ANELECTRON MICROSCOPE Gunter F. Bahr, Chevy Chase, Md'., Otmar H.Sackerlotzky, Kendall Park, N.J., and Elmar H. Zeitler, Washington,D.C., assignors to the United States of America as represented by theSecretary of the Army Filed Dec. 11, 1964, Ser. No. 417,845 6 Claims.(Cl. 250--49.5)

ABSTRACT OF THE DISCLOSURE This invention relates to electron microscopyand more particularly to a novel shutter mechanism for an electronmicroscope. The automatically operated shutter mechanism normallyobstructs passage of an electron beam when electrical current is flowingand when no current is flowing, the shutter does not obstruct thepassage of an electron beam.

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment to us ofany royalty thereon.

For quantitative evaluation of electron micrographs, it is essentialthat reproducible exposure times be obtained. However, until now thevarious mechanisms used have proved unsatisfactory for several reasons.The simplest mechanism used for controlling exposure time involvesmanually moving the plate with respect to the electron beam. This methodis employed with the Siemens electron microscope, Elmiskop I. Manualmovement of the plate is manifestly unsatisfactory when short exposuretimes are involved and even with longer eX- posure time uneven exposureover the surface of the plate occurs.

More sophisticated automatically operated mechanisms have been employed,some located at the focal plane of the microscope and others located atvarious points along the electron beam. While automatic operationincreases the speed of the mechanical movement involved in the shutter,the large cross section of the electron beam at the focal plane stillcauses exposure time variations across the image. The use of anautomatic shutter mechanism is also limited since the magnetic andelectrical fields produced by the automatic devices effect the electronbeam and distort the images produced.

It is accordingly an object of this invention to provide an automatic,quick acting shutter mechanism for an electron microscope.

It is another object of this invention to provide an automatic shuttermechanism that does not produce undesirable electric and magnetic fieldsduring the exposure period.

It is another object of this invention to provide a shutter mechanism inwhich a small shutter element moves only a short distance between thebeam obstructing and beam passing positions.

These and other objects will be readily apparent from the followingdescription with reference to the accompanying drawings wherein:

FIG. 1 is a schematic representation, partly in section, of an electronmicroscope showing the general arrangement of the elements;

3,359,418 Patented Dec. 19, 1967 FIG. 2 is a plan view of the shuttermechanism shown in FIG. 1;

FIG. 3 is a cross sectional view of the automatic plunger mechanism;

FIG. 4 is a perspective view of the sensor for determining intensity ofthe electron beam; and

FIG. 5 is an oscillogram tracing showing the electron beam intensitychange on operation of the shutter.

The invention has general utility but for purposes of illustration itwill be described with respect to the Siemens Elmiskop I the electronmicroscope in which the shutter mechanism has been used. The electronmicro scope is shown generally at 1, having a source of electrons 2, alens section 3, a sample section 4, an objective lens 6, an intermediatelens 7 and. a screen 8. In the Siemens microscope, the objective andintermediate lenses are contained within one casing or pole piece 9having a wall 10 defining an annular central passage through which theelectron beam passes. Access to the enclosed volume, Within the polepiece, between the objective and intermediate lens is bad by means ofthreaded holes in the casing, one of which is shown at 11, having theshutter actuating means 12 in threaded engagement therewith. Flexiblesealing means can be provided in the hole to prevent leakage through thehole. A hole 13 is provided in the wall 10 to permit passage of theplunger 14 of shutter actuating means 12 as will be explained more fullybelow.

A hollow cylindrical sleeve 15 is slidably mounted within the annularpassage defined by wall 10. The cylindrical sleeve 15 carries themovable shutter blade 16 and has a hole 50 aligned with the hole 13 inwall 10 to permit passage of plunger 14. As can be seen in FIG. 2shutter blade 16 is pivotally mounted on the cylindrical sleeve 15 bymeans of pin 17 which passes through a hole inthe shutter blade 16 andis mounted in the bottom face of a notch 18 in the sleeve 115. Theshutter blade is biased by resilient means such as spring 19 to normallybein a position that does not obstruct the passage of the electron beam,the locus of which is shown by the dotted circle E in the center ofcylindrical sleeve 15. Spring 19 extends around pivot pin 17 having oneleg affixed to blade 16 and the other leg abutting the edge of groove18. For convenience, the sleeve may be made of upper and lower sectionswhich are joined together after the assembly of the spring 19, pin 17and blade 16.

The shutter actuating means 12 comprises a housing 20 having anexternally threaded portion 21 on one end adapted to engage the threadsof hole 11 in the microscope pole piece housing 9. A central boreextends through the housing 20 having a small diameter section 22 in theexternally threaded end of housing 20, a larger diameter intermediateportion 23 and an overboard terminal portion 24 provided with internalthreads. Field coil 25 having a central bore rests in the intermediateportion 23 and is held in place on collar 26, which rests on the lowerface of the intermediate portion 23, by retaining ring 27 which sits ina groove cut in the wall of intermediate portion, 23. The plunger 14made of a non-magnetic material such as brass may have a narrow innerend 28, a wide intermediate portion 29 and a narrow outer end 30. Theouter end 30 fits slidably within collar 26 and is threadably engaged atits upper end with soft iron armature 31. A hearing 33 is provided inbore 22 within which slides intermediate portion 29 of the plunger 14.

A return spring 32 is connected at one end to armature 31 and at theother end to plate 34 mounted in an annular groove cut in the wall ofintermediate portion 23 of the bore adjacent the outer end thereof. Anring 35 is mounted in a groove cut into the face of overbored portion 24and a brass sealing plate 36 is pressed into contact with O ring 35 byretainer ring 37 which is threadably engaged with the internal threadsin overbored portion 24. Electrical conductors 38 and 39 are connectedto coil 25 and extend from the actuator unit 12 for connection to asource of current which may be an automatic timer 60. To maintain avacuum seal, conductors 38 and 39 pass through a rubber grommet 40extending through aligned holes in plate 34 and sealing plate 36. APlexiglas housing 41 may be mounted on the end of casting 20 to preventflexing of the conductors at grommet 40'.

In use when current is not flowing through coil 25 the plunger 14 is inits Withdrawn position and shutter blade 16 does not obstruct theelectron beam, as shown in solid lines in FIG. 2. To obstruct the beam,current is passed through coil 25, extending the plunger 14 and pushingthe blade 16 to the portion shown in dotted line in FIG. 2. When currentflow stops spring 32 pulls plunger 14 to its withdrawn position andspring 19 moves blade 16 to its non-obstructing position. As can beseen, no current flows through the solenoid during exposure of the plateto the electron beam and distortion of the image caused by straymagnetic fields created by the operation of the shutter is eliminated.The relative strength of springs 19 and 32 and the electromagneticsolenoid are not critical and can be readily determined by a Worker inthe art within the following criteria. Both return springs should besufliciently powerful with respect to the mass of the shutter andplunger to move them rapidly to their return positions. Likewise thenumber of turns in the coil and the current flow should be chosen tocreate a force on the armature that is sufliciently greater than thecombined strengths of the return springs to allow the beam to be rapidlybroken.

The length of the stroke is dependent on the dimensions of the electronmicroscope itself and upon the size of the shutter blade. The necessarystroke length is obtained by proper selection of the length of the neckof collar 26 for the power stroke and the length of narrow outer end 30of the plunger for the return stroke.

To obtain reproducible exposure it is necessary to determine the beamintensity for each exposure. A simple detector is shown in FIG. 4 andcomprises a metallic disc 43 soldered or brazed to a metallic rod 44.The rod 44 is mounted in an insulator 45 so that metallic disc 43 isslightly above the microscope screen 8. Electrical conductors 46 and 47connect the rod to a micro-microammeter 48. Since exposure is a productof beam intensity and time, the intensity readings obtained from themeter 48 can be readily used to calculate the exposure time.

FIG. illustrates the rapidity of shutter mechanism. The oscillogramtracing was obtained by placing a sensor, the size of the screen 8,connected to the oscillogram on the screen 8 and actuating the shuttermechanism for an 0.2 second exposure. As can be seen the beam intensityincreased sharply with no edge effects and decreased just as sharply.

While the invention has been described with respect to a specificelectron microscope, it is readily apparent that the shutter mechanismcan be adapted for use in any electron microscope in accordance with thefollowing principles:

First of all it is essential that the shutter mechanism be located inthe lens system at a point where the electron beam has its narrowestcross section. By so locating the shutter blade only a small mechanicalmovement is required to break the beam and the shutter itself can have asmall cross section. As a result the entire beam cross section can bebroken almost instantaneously.

Secondly the motion of the shutter should be perpendicular to the axisof the beam to reduce to a minimum the length of travel of the shutterblade.

Thirdly the shutter and actuating mechanisms must be so biased thatcurrent only flows through the solenoid when the beam is obstructedthereby preventing distortion of the image by magnetic fields duringexposure.

The invention therefore is not limited to the details of the specificdevice; rather it covers mechanical modifications necessary to utilizethe shutter in other electron miscroscopes and is limited only by thefollowing claims.

We claim:

1. In combination with an electron microscope having at least one coilfor the focusing of a beam of electrons, an electrically operatedshutter mechanism which obstructs the beam when current is flowing anddoes not obstruct the beam when current is not flowing in the shuttermechanism, said mechanism comprising:

(a) a shutter blade pivotally mounted within said electron microscope inclose proximity to a point where the electron beam cross section isnarrowest, said blade being capable of movement in a plane perpendicularto the axis of the electron beam from a position where said beam isobstructed by said blade to a position where said beam is not obstructedby said blade;

(b) biasing means coacting with said shutter blade to maintain saidblade in a position where said beam is not obstructed;

(c) actuating means abutting said shuter blade, said actuating meansbeing biased to maintain said blade in a position where said blade doesnot obstruct said electron beam; and

(d) electromagnetic means for moving said actuating means and shutterblade, against their combined biases from the point where said shutterblade does not obstruct the said beam to a point where said shutterblade obstructs said beam.

2. The combination of claim 1 wherein said electromagnetic rneanscomprises an armature and a coil having a central passage in which saidarmature slides and said actuating means is connected to said armature.

3. The combination of claim 2 further comprising:

(a) a housing having a first and second end and a central bore having anoverbored portion adjacent said second end, said overbored portion beingprovided with internal threads and said bore having an internal grooveadjacent said overbored portion;

(b) means for fixedly mounting said coil in said housing with saidactuating means extending through said first end of said bore;

(c) a backing plate mounted in said internal groove;

(d) spring means connecting said backing plate to said armature;

(e) sealing means disposed in said overbored portion of said housing;and

(f) externally threaded closure means in threaded engagement with saidinternal threads on said housing, whereby said second end of saidhousing may be. sealed.

4. The combination of claim 3 further comprising electrical conductorsconnecting the coil to a source of electrical current, said conductorspassing in vacuum sealed relationship through said backing plate andsealing means.

5. The combination of claim 3 wherein said electron microscope has acasing provided With at least one hole communicating with the interiorof said microscope adjacent that portion thereof at which the electronbeam is narrowest, said hole being provided with internal threads at theouter end thereof; and said housing is provided with 5 external threadsadjacent said first end thereof whereby said housing may be threadablyengaged with said microscope casing.

6. The combination of claim 2 wherein said actuating means isconstructed from non-magnetic material.

References Cited UNITED STATES PATENTS 3,102,194 8/1963 Van Der Broek etal. 250-495 5 ARCHIE R. BORCHELT, Primary Examiner.

RALPH G. NILSON, WILLIAM F. LINDQUIST,

A. L. BIRCH, Assistant Examiner.

Examiners.

1. IN COMBINATION WITH AN ELECTRON MICROSCOPE HAVING AT LEAST ONE COILFOR THE FOCUSING OF A BEAM OF ELECTRONS, AN ELECTRICALLY OPERATEDSHUTTER MECHANISM WHICH OBSTRUCTS THE BEAM WHEN CURRENT IS FLOWING ANDDOES NOT OBSTRUCT THE BEAM WHEN CURRENT IS NOT FLOWING IN THE SHUTTERMECHANISM, SAID MECHANISM COMPRISING: (A) A SHUTTER BLADE PIVOTALLYMOUNTED WITHIN SAID ELECTRON MICROSCOPE IN CLOSE PROXIMITY TO A POINTWHERE THE ELECTRON BEAM CROSS SECTION IS NARROWEST, SAID BLADE BEINGCAPABLE OF MOVEMENT IN A PLANE PERPENDICULAR TO THE AXIS OF THE ELECTRONBEAM FROM A POSITION WHERE SAID BEAM IS OBSTRUCTED BY SAID BLADE TO APOSITION WHERE SAID BEAM IS NOT OBSTRUCTED BY SAID BLADE; (B) BIASINGMEANS COACTING WITH SAID SHUTTER BLADE TO MAINTAIN SAID BLADE IN APOSITION WHERE SAID BEAM IS NOT OBSTRUCTED; (C) ACTUATING MEANS ABUTTINGSAID SHUTER BLADE, SAID ACTUATING MEANS BEING BIASED TO MAINTAIN SAIDBLADE IN A POSITION WHERE SAID BLADE DOES NOT OBSTRUCT SAID ELECTRONBEAM; AND (D) ELECTROMAGNETIC MEANS FOR MOVING SAID ACTUATING MEANS ANDSHUTTER BLADE, AGAINST THEIR COMBINED BIASES FROM THE POINT WHERE SAIDSHUTTER BLADE DOES NOT OBSTRUCT THE SAID BEAM TO A POINT WHERE SAIDSHUTTER BLADE OBSTRUCTS SAID BEAM.